A printed wiring board mounted on an electronic device requires a micro wiring technology and a high density mounting technology associated with miniaturization of an electronic device, while it requires a technology for high heat dissipation corresponding to heat generation. Particularly, an electronic circuit in a vehicle or the like using a large current for various types of controls/operations has a very large heat generation caused by the resistance of a conductive circuit or from a power module, and thus requires high heat dissipation characteristics besides high copper foil peel strength, heat resistance in moisture absorption, resistance to soldering heat and low water absorption which have been needed for conventional printed wiring boards.
A thermosetting resin itself such as an epoxy resin used in an insulating layer of a printed wiring board has a low thermal conduction. Hence, there is known a method by which a thermosetting resin is highly filled with an inorganic filler having excellent thermal conduction property in order to enhance the thermal conduction as a printed wiring board (see, for example, Patent Literature 1).
However, problems caused by highly filling a thermosetting resin composition with an inorganic filler are that the processability of the resin composition becomes deteriorated, that the resin composition becomes brittle or expensive, or the like. On top of that, a problem is that since the volume ratio of the thermosetting resin becomes lower, which facilitates the occurrence of cracks and voids between the resin and the inorganic filler, moisture-resistant characteristics (water absorption rate and heat resistance in moisture absorption) and resistance to soldering heat are reduced, and further the adhesion between the resin and the inorganic filler becomes insufficient, leading to reduced copper foil peel strength.
Meanwhile, as an inorganic filler having excellent thermal conduction property, boron nitride is known (see, for example, Patent Literature 2), and Examples of Patent Literature 2 use boron nitride (“UHP-2” SHOWA DENKO K.K.), which is commercially available as hexagonal boron nitride. It is known that the hexagonal boron nitride excels in electrical insulation properties and chemical stability besides thermal conduction property, and is non-toxic and comparatively inexpensive. However, it is known that the hexagonal boron nitride grows in the a-axis direction (direction of a hexagonal mesh surface), but does not grow in the c-axis direction (direction of a pile) as the particle size becomes larger. Thus, it is known that, in the case where a general silane coupling agent is used, the effect by using a silane coupling agent could not be achieved due to less functional groups with respect to the flattened surfaces. Although Patent Literature 2 reports that moisture-resistant characteristics (heat resistance in moisture absorption), which are needed for a printed wiring board use can be improved by using a silane coupling agent having a specific functional group, the technique lacks versatility and effects thereof are definite.
Further, Patent Literature 3 describes that the heat conductivity of a substrate can be improved by using magnesium and/or calcium borate particles covered with hexagonal boron nitride in place of scale-like hexagonal boron nitride. However, the resins used in Examples of the Literature are only an epoxy resin having a common structure, and the substrate obtained by curing it could not be said to have sufficient characteristics in terms of high glass transition temperature, high copper foil peel strength, heat resistance in moisture absorption, resistance to soldering heat, water absorption rate and flame resistance which are needed for a printed wiring board use.
Furthermore, in Patent Literature 4, a resin composition characterized in that either or both of an epoxy resin and a curing agent has a naphthalene structure, an inorganic filler contains hexagonal boron nitride, and the inorganic filler is contained at 50 to 85% by volume of the entire resin composition is cured to make a sheet or a substrate. However, the use of the cured material described in the Literature is a heat dissipation sheet, and thus could not be said to have sufficient characteristics in terms of high glass transition temperature, high copper foil peel strength, heat resistance in moisture absorption, resistance to soldering heat, water absorption rate and flame resistance, which are needed for a printed wiring board use.